Stock tire size is almost 11" (10.92") wide I think. So if you were looking at a 12.50, took a ruler to it, it would measure 12.5" across. Put it on the truck and its still going to measure 12.5", but you'd still only gain .75" of outside the truck...AND inside the wheel well (for an extra 1.5" total). That's also why you needs the 0 or negative offset, to get away from the control arm.
The offset is the only measurement that is going to bring the tire AND wheel out from the wheel well together. The offset numbers are just millimeters. Backspacing is just another term for it.
The biggest help that I got was googling offset and backspacing to better understand it. It's simple when it finally "clicks".
To answer your original question simply, -25 will work, but -12 on a 10" wide wheel will also work. You're not gonna start really sticking out ridiculously until you go past that -25. My 20x9 0 offset is flush with the fender flares at the highest point and sticks out around 2" on bottom. I'm imaging you might start throwing rocks all over your truck beyond a 5" backspacing because you'd be exceeding the wheel wells a good amount. It's really hard to picture this even using a tape measure on your truck which I did for months.
A lot of these wheel places will test fit things for you too sometimes. That might be the easiest way to figure it out! Good luck!
